This invention relates to a system for generating symmetrical paths in a numerical control device, which device controls the table of a machine tool or the like, in such a manner that machining along the symmetrical paths can be carried out continuously in one machining direction.
A typical conventional numerical control device comprises, as illustrated in FIG. 1, a tape reader 2 forwardly reading instruction information from a paper tape 1, a circuit 3 decoding the instruction information and producing interpolating information, an interpolating circuit 5 producing pulse series, a driving circuit 6 driving the table of a machine tool or the like, and a symmetrical path control circuit 7 reversing the moving direction in each axis of coordinates. Accordingly, when a symmetrical path is generated, the only necessary modification is to reverse the moving direction of each axis, since the direction of the revolution of a cutter does not change. Thus, for instance, when a symmetrical path, as shown in FIG. 3, is generated from a paper tape which instructs the first quadrant path, as shown in FIG. 2, it is only possible to provide said symmetrical path by switching the moving direction of a cutter, namely, from a clockwise direction (first and third quadrants) to a counterclockwise direction (second and fourth quadrants) and vice versa.
However, in a numerical control device controlling a milling machine, if the machining direction along a symmetrical path is switched, the position of a piece of work relative to the moving direction of a cutter is switched from one side to the other. Since a milling machine cutter having a regular structure cuts the work by the forward rotation of the main shaft, the aforesaid switching of the machining direction causes a switching between the upper cut and the down cut, which results in a change of cutting conditions and leads to roughening of the cut surface. In addition, when a closed path is generated by a combination of the conventional symmetrical paths, in order to follow through the closed path, as the operation of a cutter moves from one quadrant to another quadrant, the machine operation must be shifted from the end point of the one quadrant path to a separate starting point of the symmetrical path in the other quadrant.